The City of Souls by Vacen Taylor

With a new addition to their journey, a sealer boy who is bound by the collar of slavery, Mai, Long, and Akra escape the clutches of the sand slavers and then vicious Melkarie beasts. 

They travel to Naroan – the forest lands of the soulbankers, the regulators of life and death. Against the backdrop of rules and suspicion, the children are challenged with unravelling the mystery of the Silvershade, which has been calling to Akra from the moment he arrived in the forest city. 

But Long is tormented by his own doubts – he must face a deadly power from the Underworld before it takes him into the darkness.